The present invention relates to printed circuit boards of the type having a solder mask over the copper circuit traces and, more particularly, to a method of manufacture of such circuit boards and to the unique printed circuit boards resulting therefrom.
As is well understood in the art, the manufacture of double-sided printed circuit boards requires the provision of conductive through-holes for interconnecting components on opposite sides of the board or, in the case of multilayer printed circuit boards, for interconnecting the inner layers. The non-conductive surfaces exposed when through-holes are drilled in a nonconductive substrate having metal cladding on both sides must, therefore, be provided with a conductive coating, and this generally is accomplished by a first electroless deposition of copper onto the suitably conditioned through-hole surfaces, followed by electroplating of copper to build up additional thickness.
In application of the actual circuit patterns to the metal-clad board surfaces, it is necessary to employ plating resists so as to prevent all but particular areas of the board (through-holes and/or traces and/or pads and/or other areas) from receiving applied metal platings such as the copper electroplate used in through-hole plating or the commonly-employed tin-lead coating applied as an etch-resist preliminary to the step of etching away undesired metal down to the non-conductive substrate surface so as to form the appropriate conductive circuit pattern and to ensure subsequent soldering of components.
Apart from its use as an etch-resist, tin-lead is a preferred overplating for otherwise exposed copper areas on the circuit board so as to prevent oxidative degradation of the copper surfaces.
In the ultimate fabrication of a printed circuit board in which various components and connections are soldered, it is generally accepted that improved solderability of circuit pads and through-holes can be provided to the ultimate fabricator by having the manfacturer precoat these areas with a solderable metal, generally a tin-lead composite closely similar in composition to the solder actually used in the eventual soldering of components and connections. For applications where hand-soldering by the fabricator is to be performed, little difficulty is encountered in applying solder to desired areas without disturbing or inadvertently soldering adjacent conductive traces. However, when soldering is to be conducted in mass techniques, such as with wave soldering or dip soldering methods, inadvertent soldering and improper connections can occur. As a consequence, manufacturers apply a solder resist or solder mask over those areas of the board to be protected from solder, including the tin-lead coated copper traces.
It has been recognized that the technique of solder resist over tin-lead coated copper can, however, lead to its own peculiar problems. For example, since the tin-lead is a reflowable metal, ultimate wave or dip soldering can cause the tin-lead to wick up under the mask or simply to melt and no longer provide support for the mask. Due to these disadvantages, it has been proposed to apply the solder mask directly over bare copper at those areas where protection from solder is desired. This "solder mask over bare copper" (SMOBC) technique avoids the problems inherent in the application of the mask over tin-lead coated copper, and can yield printed circuit boards with finer line definition and higher circuit density capabilities. Unfortunately, the known solder mask over bare copper techniques involve added manufacturing operations, and hence added cost, and present waste disposal and pollution control problems.
It has now been found that the above problems can be overcome or significantly reduced by applying the solder mask over a film of copper oxide on the surface of the copper layer to be protected. The copper oxide film is produced in situ on the copper layer in question using a novel process, to be described hereinbelow, for stripping of the tin-lead alloy etch resist employed in the earlier step of the process of fabricating a printed circuit board. Further, it is found that the application of the solder mask to copper covered with a film of copper oxide rather than to bare copper itself results in significantly increased strength of adhesion of the solder mask to the copper layer.